1. Field of the Invention
The present invention relates to an optical attenuator, and more particularly to a comb drive actuator using electrostatic force.
2. Description of the Related Art
As well known to those skilled in the art, a drive mechanism using electrostatic force is commonly used to drive a VOA (variable optical attenuator).
A comb actuator recommended by the university of California, U.S.A is well described in U.S. Pat. No. 5,025,346. This drive mechanism is widely employed because of its relative simplicity in manufacturing process and structural characteristics compared with other drive mechanisms.
A drive mechanism disclosed in PCT Publication No. WO98/12,589 uses dual comb actuators: one used when light is cut off and the other used when light is not cut off.
A drive mechanism disclosed in U.S. Pat. No. 6,229,640 uses a single comb actuator, which uses drive force of the comb actuator and spring force of a spring to cut off or transmit light.
Today, a MEMS variable optical attenuator of shutter-type using one of above described actuators is mainly used.
The shutter-type VOA places a shutter between a pair of transmission and reception fibers, wherein the shutter is moved so as to adjust a size of a junction between two fibers, thereby controlling a level of attenuation. Assuming that light is distributed according to a Gaussian distribution of one dimension, the level of attenuation [dB unit] decreases significantly as a light block distance increases, as shown in a graph in FIG. 1. If it is assumed that light is distributed according to a Gaussian distribution of two dimensions, then the decrease in the level of attenuation becomes more severe.
The level of attenuation decreases as the light block distance increases, and is proportional to the third power of the light block distance, as shown in FIG. 1.
The light block distance is proportional to the second power of a drive voltage, as shown in the following equation 1. Hence, the level of attenuation is proportional to the fifth power of the drive voltage.                     f        =                                            ϵ              ⁢                              xe2x80x83                            ⁢                              n                c                            ⁢              t                        d                    xc3x97                      V            2                                              (        1        )                                δ        =                              f            /            k                    =                                                    ϵ                ⁢                                  xe2x80x83                                ⁢                                  n                  c                                ⁢                t                            kd                        xc3x97                          V              2                                                          (        2        )            
Here, xe2x80x9cfxe2x80x9d is a drive force generated from the comb actuator, and xe2x80x9cxcex4xe2x80x9d is a displacement. xe2x80x9cVxe2x80x9d is a voltage and xe2x80x9cxcex5xe2x80x9d is a permittivity, xcex5=8.85xc3x9710xe2x88x9212 F/m. xe2x80x9cncxe2x80x9d is the number of combs and xe2x80x9ctxe2x80x9d is a thickness of a structure while xe2x80x9cdxe2x80x9d is a gap between the combs and xe2x80x9ckxe2x80x9d is a spring constant.
In the equation 2, the displacement of the actuator is proportional to the second power of the applied voltage when other variables are constant. Let""s fix a voltage causing maximum displacement to 24V in the present invention.
The level of attenuation is proportional to the third power of the displacement xe2x80x9cxcex4xe2x80x9d, thus this can be represented by the following third-order equation 3:
Attenuation[dB]=axcex43+bxcex42+cxcex4+dxe2x80x83xe2x80x83(3)
Where, xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9ccxe2x80x9d and xe2x80x9cdxe2x80x9d are constants.
The displacement xe2x80x9cxcex4xe2x80x9d is proportional to the second power of the drive voltage, thus the level of attenuation can be represented by the following high-order equation:
Attenuation[dB]=xcex1V5+xcex2V4+xcex3V3+dxe2x80x83xe2x80x83(4)
Here, xe2x80x9cxcex1xe2x80x9d, xe2x80x9cxcex2xe2x80x9d, xe2x80x9cxcex3xe2x80x9d and xe2x80x9cdxe2x80x9d are constants.
As can be seen in the equation 4, the level of attenuation is given as a high-order, which is at least fifth-order, function of the drive voltage. Thus, a resulting graph changes more abruptly in its shape. This resulting graph is well illustrated in FIG. 2.
As described above, the conventional optical attenuator of shutter-type has a disadvantage in that a voltage-displacement characteristic due to the structure of the comb actuator and a Gaussian distribution of light wavelengths degrades linearity upon transferring data, thereby causing a performance problem.
In the structure of the conventional attenuator, the light block distance is proportional to the second power of the drive voltage, and the amount of attenuation of light increases in geometric progression with respect to the light blocking distance. Therefore, a relationship between the drive voltage and the amount of attenuation is represented by a third graph in FIG. 3.
As shown in FIG. 3, the amount of attenuation of light increases non-linearly as the light block distance increases, on account of light distributed according to a Gaussian distribution. By analyzing the first and second graphs, we can see that the amount of attenuation increases in geometric progression as the voltage increases, thereby causing a big problem in resolution.
In short, the voltage-displacement characteristic due to the structure of the comb actuator and a Gaussian distribution of light wavelengths degrades linearity upon transferring data, thereby causing a performance problem.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a variable optical attenuator having a new drive mechanism to adjust an initial position of a variable mirror or variable shutter, and an additional differential circuit in order to solve performance problems occurring in a conventional MEMS comb drive variable optical attenuator, thereby securing linearity upon transferring data.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a variable optical attenuator comprising a transmission fiber for transmitting beams of light; a reception fiber arranged on the same axis with the transmission fiber and receiving the light beams transmitted by the transmission fiber; a variable shutter arranged between the transmission and reception fibers, and moving vertically to cut off or transmit the light beams, thereby controlling an amount of attenuation, the variable shutter cutting off the light to have a maximum amount of attenuation at an initial position; a comb actuator for driving the variable shutter; and a spring for generating a spring force to return the variable shutter to its initial position, the spring force not being applied in the initial position having the maximum amount of attenuation when a drive voltage of the comb actuator is not applied, and spring force being applied when the variable shutter is driven.
In accordance with another aspect of the present invention, the above and other objects can be accomplished by the provision of a variable optical attenuator comprising a transmission fiber for transmitting beams of light; a reception fiber arranged at an angle to the transmission fiber and receiving the light beams transmitted by the transmission fiber; a variable mirror arranged between the transmission and reception fibers, and moving vertically to reflect the light beams, thereby controlling an amount of attenuation, the variable mirror reflecting the light to have the maximum amount of attenuation at an initial position; a comb actuator for driving the variable mirror; and a spring for generating a spring force to return the variable mirror to its initial position, the spring force not being applied in the initial position having the maximum amount of attenuation when drive voltage of the comb actuator is not applied, and being applied when the variable mirror is driven.